The present invention relates to a new process for the production of aliphatic oligo-carbonate diols from aliphatic diols by a multistage transesterification with dimethyl carbonate (DMC) with an almost complete consumption of the carbonate that is used. The process according to the invention enables a particularly high-yield production of aliphatic oligocarbonate diols to be achieved starting from easily accessible DMC.
Aliphatic oligocarbonate diols have been known for a long time as important intermediate products, for example in the production of plastics, lacquers and adhesives, for example by reaction with isocyanates, epoxides, (cyclic) esters, acids or acid anhydrides. They can be obtained in principle from aliphatic diols by reaction with phosgene (e.g. DE-A 1 595 446), bis-chlorocarbonic acid esters (e.g. DE-A 857 948), diaryl carbonates (e.g. DE-A 1 915 908), cyclic carbonates (e.g. DE-A 2 523 352: ethylene carbonate) or dialkyl carbonates (e.g. DE-A 2 555 805).
Of the carbonate sources, diphenyl carbonate (DPC) belonging to the diaryl carbonates is of particular importance since aliphatic oligocarbonate diols of particularly high quality can be produced from DPC (e.g. U.S. Pat. No. 3,544,524, EP-A 292 772). In contrast to for example aliphatic carbonate sources, DPC reacts quantitatively with aliphatic OH groups so that, after removal of the phenol that is formed, all terminal OH groups of the oligocarbonate diol are available for reaction with for example isocyanate groups. In addition only very small concentrations of soluble catalyst are required, with the result that the latter can remain in the product.
The processes based on DPC have the following disadvantages however:
Only ca. 13% of the DPC remains in the product, the remainder being distilled off as phenol. Depending on the respective alkyl radical, a substantially higher proportion of the dialkyl carbonates remains in the subsequent product. For example, ca. 31% of the dimethyl carbonate (DMC) remains in the subsequent product, since the methanol that is distilled off has a substantially lower molecular weight than phenol.
Accordingly, due to the fact that high boiling point phenol (normal boiling point: 182xc2x0 C.) has to be separated from the reaction mixture, only diols having a boiling point that is considerably above 182xc2x0 C. can be used in the reaction in order to avoid the diol being unintentionally distilled off.
Dialkyl carbonates, in particular dimethyl carbonate (DMC), as starting components are characterised by a better availability on account of their ease of production. For example, DMC can be obtained by direct synthesis from MeOH and CO (e.g. EP-A 0 534 454, DE-A 19 510 909).
Numerous patent applications (e.g. U.S. Pat. No. 2,210,817, U.S. Pat. No. 2,787,632, EP-A 364 052) relate to the reaction of dialkyl carbonates with aliphatic diols:
It is known from the state of the art to mix aliphatic diols together with a catalyst and the dialkyl carbonate (e.g. diethyl carbonate, diallyl carbonate, dibutyl carbonate) and distil off the alcohol that is formed (ethanol, butanol, allyl alcohol) from the reaction vessel through a column. The higher boiling point, co-evaporated dialkyl carbonate is separated in the column from the lower boiling point alcohol and is recycled to the reaction mixture.
In contrast to DPC, dialkyl carbonates do not react quantitatively with aliphatic OH groups since the transesterification of two aliphatic alcohols involves an equilibrium reaction. Thus, after the removal of the alcohol that is formed a proportion of the desired terminal OH groups are present not as OH groups but as alkoxycarbonyl terminal groups (xe2x80x94OC(O)xe2x80x94OR2 group in formula I, wherein R2 denotes an alkyl radical and R1 denotes an alkylene radical). 
These alkoxycarbonyl terminal groups are unsuitable for further reaction with for example isocyanates, epoxides, (cyclic) esters, acids or acid anhydrides. The reaction is therefore completed by applying a vacuum in order to de-cap and remove the alcohol that is formed. The reaction mixtures are normally heated and stirred in vacuo in order to achieve this objective, although the quality of oligocarbonate diols that can be achieved is not as good as is obtained by reaction with DPC.
EP-A 0 364 052 describes for example a process in which a degree of utilisation of the terminal OH groups of only ca. 97% is achieved at 200xc2x0 C. and under a vacuum of ca. 50 Torr (ca. 66 mbar). Even under considerably more drastic conditions the degrees of utilisation of the terminal OH groups can be increased only insignificantly. At 1 Torr (ca. 1.3 mbar) degrees of utilisation of only ca. 98% are achieved (EP-A 0 798 328).
The use of dimethyl carbonate (DMC) to produce aliphatic oligocarbonate diols has been known only for a fairly short time despite its good accessibility (e.g. U.S. Pat. No. 5 171 830, EP-A 798 327, EP-A 798 328, DE-A 198 29 593).
When using DMC to produce oligocarbonate diols low boiling point azeotropic DMC-methanol mixtures are formed that contain, depending on the pressure, ca. 20 to 30 wt. % of DMC (ca. 30 wt. % at normal pressure). A relatively large effort and expenditure is required to separate these azeotropic mixtures into methanol and DMC (e.g. membrane separation). The DMC that is azeotropically distilled off is accordingly lost to the reaction and is no longer available for a complete conversion. The lost DMC therefore has to be replenished by additional fresh DMC.
In EP-A 0 358 555 and U.S. Pat. No. 4,463,141 it is for example in addition simply recommended to take into account, during the weighing in, the amount of DMC that is azeotropically distilled off.
In EP-A 0 798 328 the corresponding diol component is reacted with DMC accompanied by distillation of the azeotropic mixture. The subsequent de-capping takes place under vacuum distillation, whereby under very drastic vacuum conditions (I Torr, ca. 1.3 mbar) degrees of utilisation of the terminal OH groups of ca. 98% can be achieved (EP-A 0 798 328: Table 1). No details of the remaining azeotropic mixture and the loss of the DMC are given.
EP-A 798 327 describes a two-stage process in which a diol is first of all reacted with an excess of DMC with distillation of the azeotropic mixture to form an oligo-carbonate whose terminal OH groups are completely inaccessible, being methoxycarbonyl terminal groups. After removal of the catalyst and distillation of the excess DMC in vacuo (65 Torr, 86 mbar), the oligocarbonate diol is obtained in a second stage by adding further amounts of the diol and a solvent (e.g. toluene) as entrainment agent for the methanol that is formed. Solvent residues then have to be distilled off in vacuo (50 Torr, 67 mbar). The degree of utilisation of the terminal OH groups according to this process is only ca. 97%. The disadvantage of this process is that it is complicated due to the use of a solvent and due to the multiple distillation, low degree of utilisation of the terminal OH groups, as well as the very high DMC consumption.
In DE-A 198 29 593 a diol is reacted with DMC, the methanol that is formed being distilled off. This publication does not give any details of the overall azeotropic distillation procedure, apart from a single mention of the word xe2x80x9cazeotropexe2x80x9d in the Table xe2x80x9cFlow chart of the process according to the inventionxe2x80x9d. Claim 1c states that the molar ratio of methanol to DMC in the distillate is between 0.5:1 and 99:1. The DMC content in the methanol that is distilled off is accordingly between 85 wt. % and 2.8 wt. %. As a detailed analysis shows (see below), in DE-A 198 29 593 DMC is in fact also used in excess and is distilled off azeotropically. Accordingly, ca. 27.8% of the DMC that is used is lost.
As Comparison Example 1 shows (see below), DMC contents in the distillate of less than 20% can be achieved only at high catalyst concentrations (ca. 0.15% Ti(O-iPr)4, corresponding to ca. 250 ppm Ti) and very long reaction times. At these high catalyst concentrations the catalyst cannot be left in the product after the end of the reaction, but has to be neutralised. In DE-A 198 29 593 the catalyst (Example 1: 0.15% Ti(O-nBu)4 and Example 2: 0.12% Ti(O-nBu)4) is neutralised or masked by adding phosphoric acid.
The DMC content in the distillate increases with falling catalyst concentration (Comparison Example 1). Consistently low DMC contents in the distillate can be achieved only by drastically increasing the reaction time. With a reduced catalyst concentration of ca. 0.01% Ti(O-iPr)4 (ca. 16 ppm Ti) the catalyst can remain in the product after the end of the reaction. As Comparison Example 1 shows, this leads however to industrially no longer practicable reaction times respectively DMC contents of 22 to 30% in the distillate.
In DE-A 198 29 593 no details are given concerning the degree of utilisation of the terminal OH groups.
In U.S. Pat. No. 5,171,830 butanediol-1,4 is first of all heated with DMC under reflux and then the volatile constituents are distilled off (azeotropically). After vacuum distillation under drastic conditions (1 Torr, 1.3 mbar), taking up the product in chloroform, precipitating the product with methanol and drying the product, an oligocarbonate diol is obtained in 55% of the theoretical yield (Example 6). No details are given concerning the degree of utilisation of the terminal OH groups and the azeotropic distillation procedure.
German Patent Application 1999 00 554.0 describes a process in which the transesterification of the diol with DMC is carried out by reactive rectification in a gas-liquid countercurrent apparatus. Due to the countercurrent procedure the methanol-DMC azeotropic mixture can be avoided and a DMC conversion of ca. 95% can be achieved. In order to de-cap the OH terminal groups nitrogen is passed through the product as stripping gas under a low vacuum (ca. 150 mbar) (2 to 200 Nl/h (Nl=normal liter)). By means of the stripping the methanol can be largely removed, the transesterification can be completed, and degrees of utilisation of the terminal OH groups of ca. 99.8% can be achieved.
None of the previously known publications describe industrially easily realisable processes for reacting DMC with aliphatic diols to form oligocarbonate diols with high space-time yields, almost complete conversion and with high degrees of utilisation of the terminal OH groups. The inevitable occurrence of DMC-methanol mixtures of varying composition and the associated loss of DMC considerably reduce the attractiveness of the described processes.
The object of the invention is accordingly to provide a simple, high-yield process that can also be carried out on an industrial scale, that enables oligocarbonate diols to be produced by transesterification of aliphatic diols with dimethyl carbonate, optionally with the use of such a small amount of catalyst that this can remain in the product after the end of the reaction, with good space-time yields, in simple apparatus, and with almost complete utilisation of the carbonate that is employed.
It has now surprisingly been found that the production of aliphatic oligocarbonate diols can be successfully achieved by reacting aliphatic diols with dimethyl carbonate, optionally accelerated by catalysts, with a degree of conversion of the DMC that is used of more than 80%, wherein in a multistage process DMC-methanol mixtures that have been distilled off are recycled to the reaction solution with further conversion and depletion of the contained DMC.
The present invention accordingly provides a process for the production of aliphatic oligocarbonate diols by reacting aliphatic diols with dimethyl carbonate, optionally accelerated by catalysts, with a degree of conversion of the DMC that is used of more than 80%, characterised in that in a multistage process DMC-methanol mixtures that have been distilled off are recycled to the reaction solution with further conversion and depletion of the contained DMC in the same or in a subsequent reaction batch.
In the execution of the process according to the invention for producing aliphatic oligocarbonate diols by reacting aliphatic diols with dimethyl carbonate, the mixture of DMC and MeOH that has been distilled off in a batch is re-used at the start of a following reaction batch. This DMC-methanol mixture that has been distilled off is thus re-used with further conversion of the contained DMC. Accordingly, the DMC of the previously collected and re-used mixture is depleted, and distillates are formed having reduced DMC contents.
The process according to the invention can be carried out as a two-stage or multi-stage process.
In order to achieve an almost complete conversion of the employed DMC in a multi-stage procedure, the process according to the invention is carried out as follows:
In a for example two-stage batch procedure the respective diol component is added to the vessel together with a catalyst in the first stage, and the (for example azeotrope) DMC-methanol mixture that has been formed and collected during a preceding batch process is added slowly thereto, preferably under the surface, for example through a dip tube. Depending on the feed rate, a distillate containing DMC in an amount of between 0.5 and 20%, preferably between 1.5 and 10%, and particularly preferably between 3 and 7%, is obtained at the head even of a short column.
In the second stage the residual amount of DMC, which results from the amount of DMC predetermined by the stoichiometry of the desired end product and the amount of DMC already added to the first stage of the reaction, is rapidly fed into the vessel and the DMC-methanol mixture thus distilled off over a large column (e.g. azeotropically) is collected.
The composition of all distillates is determined, and the loss of DMC that has occurred due to the distilling off of the DMC-methanol mixtures in the first and second stages is replenished in a subsequent stage by adding pure DMC. The distilled-off azeotrope consisting of DMC and MeOH is also collected and re-used in the first stage together with the distillate from the second stage in a subsequent reaction batch.
A de-capping of the terminal groups is necessary in order to achieve a degree of utilisation of the OH terminal groups of  greater than 99%.
In order to de-cap the terminal groups (utilisation of the terminal OH groups), the last residues of methanol and traces of dimethyl carbonate can be removed from the product. For example by passing in an inert gas (e.g. N2) into the oligocarbonate diol, gas bubbles are generated in the product, optionally at an only slight vacuum of e.g. ca. 150 mbar, that in the product are saturated with methanol and/or DMC. The methanol is thus almost completely expelled from the reaction batch. By stripping with an inert gas the equilibrium can be displaced still further in favour of the product by the removal of the methanol, the esterification can be brought to completion, and thus the terminal groups can be utilised. The quality of the resultant oligocarbonate diol can thus be raised to the level of DPC-based oligocarbonate diols, and the degree of utilisation of the terminal OH groups rises to more than 98%, preferably to 99.0 to 99.95%, and particularly preferably to 99.5 to 99.9%.
The distillates with the low DMC contents can be discarded, used as solvents or wash liquids in other processes, converted by aqueous hydrolysis into methanol and used further or thermally exploited as such, or can also be used in the process according to the invention in a multistage procedure with further depletion of the DMC content.
In a three-stage variant of the process according to the invention for example these mixtures may be used as follows: a ca. 5% DMC-methanol mixture that has been collected in the second stage of the preceding reaction batch is used for example in a first stage. A further depletion of the DMC in the distillate to 0.3% to 5%, preferably to 0.8% to 4%, particularly preferably to 1.5% to 3%, is thereby achieved. These distillates are discarded orxe2x80x94as previously describedxe2x80x94are utilised further. The DMC-methanol mixture containing for example ca. 30% DMC that was formed in the previous batch in the third stage is used in the second stage. A distillate containing for example ca. 5% DMC is then obtained in this case. This distillate is used in the first stage in the next batch. In the third stage pure DMC is added to the reactor, a DMC-methanol mixture containing for example 30% DMC again being formed, which is employed in the second stage of the following batch. The amount of DMC of the third stage is chosen so that the sum of the DMC amounts of all three stages after distilling off the DMC-methanol mixtures then corresponds to the amount predetermined by the desired stoichiometry. In the three-stage procedure it is therefore possible to achieve an almost quantitative utilisation of the employed DMC by recycling the distillate twice.
By appropriate repetition the process can also be carried out in more than three stages, and in fact up to n stages (where n is an integer greater than or equal to 2).
In principle a discontinuous batch procedure or a continuous procedure is possible according to the process of the invention. The batch procedure described above is only one example and should not be understood as restrictive. The person skilled in the art knows in principle how to carry out such processes in a fully continuous manner.
The addition of the DMC-methanol mixture and/or of the pure DMC can be effected in the process according to the invention also by repeated repumping of the distillate: distillates forming during the metering in procedure are returned to the pump vessel, where they are collected and reintroduced to the reactor. DMC-methanol mixture or pure DMC is thus continuously metered into the reactor from a pump vessel, a DMC-depleted mixture being distilled off and collected in the same vessel. The DMC concentration of the mixture in the vessel therefore constantly falls. In this connection the metering in rate is chosen to be higher by a multiple (e.g. ca 4 to 10 times higher) than in a simple metering in procedure. When the DMC content of the distillate has fallen to the desired value, the further addition of the DMC-methanol mixture or of the DMC to the reactor is stopped and the mixture is distilled further until the total amount of DMC and methanol has been distilled off and collected in the pump vessel.
Two vessels may also be employed when repumping the DMC-methanol mixtures or the DMC: a DMC-methanol mixture or pure DMC is passed from vessel 1 at a multiple rate (e.g. ca 4- to 10-fold rate) into the reactor and the distillate formed is collected in vessel 2. On account of the higher pumping rate this mixture does not reach the low DMC contents as previously, but instead is only somewhat depleted in DMC (for example ca. 10% to 28% depending on the catalyst concentration when using a ca. 32% DMC-methanol mixture Example 8). After all the DMC-methanol mixture or the DMC has been fed from vessel 1 into the reactor, the DMC-methanol collected in vessel 2 is passed, under the surface, into the reactor. The distillate that is now formed is collected in vessel 1. The change of vessels is repeated until the DMC content of the distillate has fallen below a desired value (for example ca. 3-5%). The distillates are thus fed under an in each case smaller depletion of the DMC, and thus more frequently, into the reactor.
In a further variant of the process according to the invention the last stage (for example the second stage in a two-stage process), in which pure DMC is rapidly metered in under distillation of the azeotropic mixture, is carried out in two partial stages: in the first partial stage the pure DMC is metered in sufficiently slowly so that not the azeotrope is distilled off, but instead a DMC-methanol mixture with for example ca. 5 to 8% DMC. This distillate isxe2x80x94like the distillate of the first stagexe2x80x94either discarded or, as described previously, used further. As Comparison Example 1 shows, the DMC content of the distillate increases with increasing reaction time on metering in the DMC. When a certain threshold value is exceeded, the remaining DMC is then added so quickly in the second partial stage that azeotropic DMC-methanol mixtures are distilled off. These mixtures are then collected and used further in a following reaction batch. The other stages in which DMC-methanol mixtures are introduced may likewise be carried out in several partial stages with different feed rates.
The process according to the invention (reaction and distillation under the addition of DMC-methanol mixtures or DMC) is in principle carried out under a light vacuum, under normal pressure, or at elevated pressure. The reaction is preferably carried out a pressure of 0.4 to 100 bar, preferably 0.7 to 15 bar, particularly preferably at a pressure of 1 to 6 bar andxe2x80x94depending on the respective pressurexe2x80x94at temperatures of 100 to 300xc2x0 C., preferably at temperatures of 160 to 240xc2x0 C. In this connection an elevated pressure leads, on account of the better azeotropic point (e.g. ca. 20% DMC/80% MeOH at 4 bar) to a better conversion of DMC and thus shorter reaction times, and also to lower DMC contents in the distillate.
The DMC content of the distillate when using a DMC-methanol mixture or pure DMC depends in each case on the feed rate and reaction time, and on the amount of catalyst: an increase in the catalyst concentration and/or a reduction in the feed rate of the DMC-methanol mixture or of the DMC (increase in the reaction time) leads to a reduction of the DMC content in the distillate. A lowering of the catalyst concentration and/or a reduction in the reaction time results in a higher DMC content in the distillate.
The amount of DMC that has been removed from the reaction batch by distillation is determined by measuring the DMC contents of the individual distillates. This missing amount must be added in the form of pure DMC to the batch before stripping the methanol with inert gases under a vacuum in order to utilise the terminal groups. A mixture of DMC and methanol is again formed. This lost DMC is replenished, a proportion being distilled off again. With each new addition the amount of DMC distilled off becomes less, and accordingly the desired stoichiometry is approached (Example 2). This complicated procedure can be simplified considerably by combining the individual subsequent stages: the amounts of DMC that are distilled off in the individual subsequent stages are known or may be calculated beforehand from previous batchesxe2x80x94for example in the first batchxe2x80x94so that the complete amount of DMC can be added in a single stage (Example 3, composition of the second stage and subsequent stages).
A small amount of DMC is lost when inert gas bubbles are pumped in during the distillation of the methanol and the de-capping of the OH terminal groups at the end of the reaction. This amount must be taken into account beforehand in the addition of DMC. This amount may be determined from the empirical values of the previous batches.
Alternatively, a small excess of DMC may be added beforehand so that, after the distilling off of the azeotrope and after the de-capping by stripping of the last residues of methanol and DMC by passing in an inert gas (e.g. N2) under a slight vacuum (ca. 150 mbar), a slight excess of DMC remains in the product or is bound as ester. After the stripping a product is thus obtained that exhibits a complete functionality of the terminal OH groups but in which the degree of polymerisation is too high. The correction is then made by adding a further amount of the diol component and carrying out a new, short esterification stage (Example 4). The correction amount may be determined on the one hand via the mass balancexe2x80x94determination of the DMC amounts in all distillates and comparison with the total amount addedxe2x80x94or may be determined from a measurable property (e.g. OH number, viscosity, average molecular weight, etc.) of the product whose degree of polymerisation is too high. A renewed de-capping is not necessary after the correction since all terminal OH groups were already free before the correction and no renewed capping is caused by adding the diol components.
A correction by adding DMC after the de-capping by gassing with an inert gas in the case of a product containing too little DMC leads to a renewed build-up of the capping.
The process according to the invention thus comprises the following process stages in the two-stage variant:
addition of the diol components and optionally the catalyst to the reactor,
1st stage: introduction of the DMC-methanol mixture (for example the azeotrope) from the previous batch and reaction of the DMC contained therein, distilling off of a DMC-methanol mixture withxe2x80x94depending on the reaction conditionsxe2x80x94for example 3 to 7% of DMC, or if desired multiple pumping of the distillate until the DMC content has fallen to the desired value,
2nd stage: introduction and reaction of pure DMC. The amount of DMC is chosen so that, after distilling off, exactly the required amount of DMC or alternatively a slight excess remains in the reaction solution in all stages (addition of the DMC-methanol mixture (e.g. azeotrope), addition of DMC and de-capping). If desired the complete amount of DMC may be metered in rapidly in one stage or in two partial stages: in the first case a DMC-methanol mixture (e.g. the azeotrope) is distilled off, collected, and re-used in the first stage in a following batch. In the second case the DMC is metered in sufficiently slowly in the fist partial stage that DMC-methanol mixtures with low DMC contents are obtained, and in the second partial stagexe2x80x94after an increase of the DMC content in the distillatexe2x80x94the DMC is then rapidly metered in so that a DMC-methanol mixture having a higher DMC content (e.g. the azeotrope) is formed, which is re-used in a following batch,
optional de-capping: utilisation of the terminal OH groups by discharging the last methanol and/or DMC residues, for example by generating gas bubbles (for example introduction of inert gases such as N2), for example under a slight vacuum (e.g. ca. 150 mbar),
optional correction: correction of the stoichiometry by adding further amounts of the diol components and renewed brief esterification.
In the first batch, in which no DMC-methanol mixtures are yet available from the previous batches, in principle only pure DMC may be used, with the result that on distillation only a DMC-methanol mixture (e.g. the azeotrope) is formed that is re-used in the first stage in the second batch, or if desired a DMC-methanol mixture (e.g. the azeotrope) is prepared by mixing DMC and methanol in the expected amounts.
Aliphatic diols with 3 to 20 C atoms in the chain are used in the process according to the invention. The following compounds may be mentioned by way of example, although this is not a complete list: 1,7-heptanediol, 1,8-ocatanediol, 1,6-hexanediol, 1,5-pentanediol, 1,4-butanediol, 1,3-butanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, 3-methyl-1,5-pentanediol, 2-methyl-pentanediol, 2,2,4-trimethyl-1,6-hexanediol, 0.3,3,5-trimethyl-1,6-hexanediol. 2,3,5-trimethyl-1,6-hexanediol, cyclo-hexanedimethanol, etc. as well as mixtures of various diols.
Furthermore the addition products of the diols with lactones (ester diols) such as for example caprolactone, valerolactone, etc., may be used as well as mixtures of the diols with lactones, an initial esterification of lactone and the diols not being necessary.
Moreover there may be used the addition products of the diols with dicarboxylic acids, such as for example: adipic acid, glutaric acid, succinic acid, malonic acid, etc. or esters of dicarboxylic acids as well as mixtures of diols and dicarboxylic acids or esters of dicarboxylic acids, a preliminary transesterification or dicarboxylic acid and diols not being necessary.
Polyether polyols may furthermore be used, such as for example polyethylene glycol, polypropylene glycol, polybutylene glycol as well as polyether polyols that have been obtained by copolymerisation of for example ethylene oxide and propylene oxide, or polytetramethylene glycol that has been obtained by ring-opening polymerisation of tetrahydrofuran (TBF).
Mixtures of various diols, lactones and dicarboxylic acids may be used.
1,6-hexanediol, 1,5-pentanediol and mixtures of 1,6-hexanediol and caprolactone are preferably used in the process according to the invention.
As catalysts there may in principle be used all soluble catalysts known for transesterification reactions (homogeneous catalysis), as well as heterogeneous transesterification catalysts. The process according to the invention is preferably carried out in the presence of a catalyst.
Particularly suitable for the process according to the invention are hydroxides, oxides, metal alcoholates, carbonates and organometallic compounds of metals of main groups I, II, III and IV of the Periodic System of the Elements, of subgroups III and IV, as well as the elements of the rare earth group, in particular compounds of Ti, Zr, Pb, Sn and Sb.
The following may be mentioned by way of example: LiOH, Li2CO3, K2CO3, KOH, NaOH, KOMe, NaOMe, MeOMgOAc, CaO, BaO, KOt-Bu, TiCl4, titanium tetraalcoholates or terephthalates, zirconium tetraalcoholates, tin octanoates, dibutyltin dilaureate, dibutyltin oxide, dibutyltin methoxide, bistributyltin oxide, tin oxalates, lead stearates, antimony trioxide, zirconium tetra-iso-propylate, etc. Inorganic or organic acids may furthermore be used as catalysts, for example phosphoric acid, acetic acid, p-toluenesulfonic acid.
There may furthermore be used in the process according to the invention tertiary amines R1R2R3N where R1-3 denotes C1-C30-hydroxyalkyl, -aryl or -alkyl, in particular trimethylamine, triethylamine, tributylamine, N,N-dimethylcyclohexylamine, N,N-dimethylethanolamine, 1,8-diazabicyclo-(5.4.0)undec-7-en, 1,4-diazabicyclo-(2.2.2)-octane, 1,2-bis(N,N-dimethylamino)ethane, 1,3-bis(N,N-dimethylamino)propane and pyridine.
Preferably the alcoholates and hydroxides of sodium and potassium (NaOH, KOH, KOMe, NaOMe), the alcoholates of titanium, tin or zirconium (e.g. Ti(OPr)4), as well as organotin compounds are used, the titanium, tin and zirconium tetra-alcoholates preferably being used with diols that contain ester functions or mixtures of diols with lactones.
In the process according to the invention the homogeneous catalyst is optionally used in concentration (specified in weight percent of metal referred to aliphatic diol that is used) of up to 1000 ppm (0.1%), preferably between 1 ppm and 500 ppm, particularly preferably 5 to 100 ppm. The catalyst may be left in the product after the end of the reaction or may be separated, neutralised or masked. The catalyst is preferably left in the product.
The removal of the methanol for the de-capping of the terminal groups may take place for example by heating the reaction mixture in vacuo, preferably by producing gas bubbles in the apparatus. These gas bubbles may be generated by passing inert gases such as nitrogen, argon, methane, ethane, propane, butane, dimethyl ether, dry natural gas or dry hydrogen into the reactor, wherein the methanol-containing and dimethyl carbonate-containing gas stream leaving the oligocarbonate may be added again to the oligocarbonate for the saturation.
These gas bubbles may also be produced by passing in inert, low boiling point liquids such as pentane, cyclopentane, hexane, cyclohexane, petroleum ether, diethyl ether or methyl-tert.-butyl ether, wherein the substances may be passed in in liquid or gaseous form and the methanol-containing and dimethyl carbonate-containing gas stream leaving the oligocarbonate may be partially added again to the oligocarbonate for the saturation. Preferably nitrogen is used.
The substances used to produce gas bubbles may be added to the oligocarbonate through simple dipping tubes, preferably by means of annular nozzles or gassing stirrers. The degree of utilisation of the terminal OH groups that is achieved depends on the duration of the de-capping, and on the amount, size and distribution of the gas bubbles: with increasing duration of the de-capping and better distribution (for example better distribution and larger interface due to larger numbers of smaller gas bubbles when the latter are introduced through a gassing stirrer), the degree of utilisation is improved. When introducing for example nitrogen (150 mbar, 40 Nl/h) through a gassing stirrer, after one hour a degree of utilisation of ca. 99% is achieved, and after 5 to 10 hours a degree of utilisation of 99.8% is achieved.
The de-capping by producing inert gas bubbles in the oligocarbonate diol is carried out at temperatures of 130xc2x0 C. to 300xc2x0 C., preferably at temperatures of 200xc2x0 C. to 240xc2x0 C., and at pressures of 0.01 to 1000 mbar, preferably at pressures of 30 to 400 mbar, particularly preferably at pressures of 70 to 200 mbar.
The molecular weight of the oligocarbonate diols produced by the process according to the invention is adjusted via the molar ratio of diol to DMC, the molar ratio of diol to DMC being between 1.01 and 2.0, preferably between 1.02 and 1.8, and particularly preferably between 1.05 and 1.6. The specified ratio describes of course the stoichiometry of the product, i.e. the effective ratio of diol to DMC after distilling off the DMC-methanol mixtures. The amounts of DMC that are used in each case are correspondingly higher due to the azeotropic distillation of the DMC. The calculated molecular weights of the oligocarbonate diols produced by the process according to the invention are then, for example in the case of 1,6-hexanediol as diol component, between 260 and 15000 g/mole, preferably between 300 and 7300 g/mole, particularly preferably between 350 and 3000 g/mole. If a diol of heavier or lighter molecular weight is used, then the molecular weights of the oligocarbonate diols produced according to the invention are correspondingly higher or lower.
The process according to the invention enables oligocarbonate diols of the formula II to be produced having between 7 and 1300, preferably between 9 and 600 and particularly preferably between 11 and 300 carbon atoms in the chain, in which R1 is used as a symbol for aliphatic diols with between 3 and 50, preferably between 4 and 40, and particularly preferably between 4 and 20 carbon atoms in the chain. 
The diols may additionally contain ester, ether, amide and/or nitrile groups. Preferred are diols or diols with ester groups, such as are obtained for example by the use of caprolactone and 1,6-hexanediol. If two or more diol components are used (for example mixtures of various diols or mixtures of diols with lactones), then two adjacent groups R1 in a molecule may be completely different from one another (statistical distribution).
The process according to the invention permits the reproducible production of high-grade oligocarbonate diols from DMC with good space-time yields under high conversion rates of the DMC.
The oligocarbonate diols produced by the process according to the invention may be used for example to produce plastics materials, fibres, coatings, lacquers and adhesives, for example by reaction with isocyanates, epoxides, (cyclic) esters, acids or acid anhydrides. They may also be used as binders, binder constituents and/or reactive diluents in polyurethane coatings. They are suitable as structural elements for moisture-hardening coatings, and as binders or binder constituents in solvent-containing or aqueous polyurethane coatings. They may furthermore be employed as structural elements for polyurethane prepolymers containing free NCO groups, or in polyurethane dispersions.
The oligocarbonate diols produced by the process according to the invention may also be used to produce thermoplastics materials such as aliphatic and/or aromatic polycarbonates, thermoplastic polyurethanes, etc.